1. Field of the Invention
The present invention relates to an aerosol size-selective impactor for reducing particle bounce. Particularly, it relates to an aerosol size-selective impactor for reducing particle bounce that has a buffering layer and a dehydration preventing layer. It also utilizes the design of the receiving cavity to contain both layers. This invention has the advantages and functions as follows. Its accuracy is high. Its sampling time lasts longer. It can endure vibrations. It can minimize the particle bounce effect. Its cost is low. And, its application scope is wide.
2. Description of the Prior Art
In the past, the aerosol sampling at a working place or environment is to measure the total amount of the airborne aerosol particles. However, the hazardous aerosols will stay in different locations in human's breathing system. However, due to the size discrepancy of some aerosols, they will not reach the deeper area of the respiratory organ. For example, aerosols with a sufficiently large aerodynamic diameter having enough inertia force have little chance of entering the alveolar region of the lungs. Therefore, when a person measures the exposure amount of the ambient aerosols, the particle size distribution as a function of aerodynamic diameter should be considered. The respiratory tract can be treated as three regions: the head airways region, the tracheobraonchial region of the lungs including the trachea and ciliated airways, and the alveolar region of the lungs including non-ciliated airways and alveolar sacs. Using size-selective sampling recommendations as a basis, three fractions of the ambient airborne particles are defined: inspirable, thoracic and respirable.
These relate to airborne particles which can be expected to enter the head, lungs and alveolar regions of the lungs, respectively. The 50% cut-off size of the respirable, thoracic and inspirable fraction is 4 μm, 10 μm and 100 μm, respectively.
Based on the above-mentioned reasons, it is necessary to develop a size-selective impactor that can conduct a more precise aerosol sampling.
For the labor, the purpose for aerosol sampling at a working environment is to evaluate the health hazards associated with particles deposited in the human breathing system. For example, it can evaluate the mass concentration and size distribution of the airborne particles. An ideal size-selective sampler not only can provide a better aerosol sampling, but also can precisely evaluate the influence on human's health. The basic operation of a typical size-selective sampler is described below. Aerosols are passed through a nozzle and the output stream directed against a flat plate. The flat plate, called an impaction plate, deflects the flow to form an abrupt 90-degree bend in the streamlines. Particles whose inertia exceeds a certain value are unable to follow the streamlines and impact on the impaction plate. Smaller particles which can follow the streamlines and avoid hitting the impaction plate remain airborne and flow out of the impactor. All particles that impact on the impaction surface must stick on it without bounce. The particles collected from the impaction surface can be either chosen for subsequently analysis or discarded if treat impactor as a pre-classifier, for different sampling purposes and impactor designs.
Referring to FIG. 1, it is a traditional impaction substrate 90 that is coated with oil or grease. Practically, it is the silicone oil 91. This coating surface with oil or grease can increase the adhesion energy, the deformation and the dissipated energy (for the aerosols 92) on the surface of the impaction substrate 90. So, the dynamic energy of the moving particles can be reduced. It also can minimize the particle bounce problem. This kind of coating surface not only can reduce the particle bounce, but also can lower the particle loading effect.
As shown in FIGS. 2A and 2B, they illustrate the process when the moving aerosols 92 hit on the silicone oil 91. These aerosols 92 will stay inside the silicone oil 92. Because these aerosols 92 will not be dissolved in the silicone oil 91, they will stay and accumulate. When the loading of the hitting aerosols 91 exceed a certain level, as shown in FIG. 2C, the moving aerosols 92 will hit the prior laden aerosols 92 and bounce off eventually. This is the typical particle bounce problem. This particle bounce will significantly reduce the accuracy of the sampling data. Moreover, assuming that there are 1000 aerosols 92, only the first 500 aerosols 92 hit the silicone oil 91 and stay inside. So, the other 500 aerosols 92 will bounce away. Under such circumstance, the error or sampling bias of this measurement becomes extremely large.
Furthermore, the oil or grease used in the traditional size-selective sampler usually is the silicone oil 91. However, the silicone oil 91 cannot sustain long-term heavy particle loading as incoming particles are bouncing off particles that had already been deposited. Its sampling time cannot last too long. It is another disadvantage.
In addition, if the silicone oil 91 is replaced by another oil with low volatility, its viscosity is lower. So, it is very easy to leak out while this sampler is shaking, vibrating or tilting (especially when the person is walking). Consequently, the sampling result will be inaccurate.